Lawrence H. Thaller

Electrolyte Management Considerations in Modern Nickel Hydrogen and Nickel Cadmium Cell and Battery Designs

In the early 1980s, the battery group at the NASA Lewis Research Center (LeRC) reviewed the design issues associated with nickel/hydrogen cells for low-earth orbit applications. In 1984, these issues included gas management, liquid management, plate expansion, and the recombination of oxygen during overcharge. The design effort by that group followed principles set forth in an earlier LeRC paper that introduced the topic of pore size engineering. Also in 1984, the beneficial effect of lower electrolyte concentrations on cycle life was verified by Hughes Aircraft as part of a LeRC-funded study. Subsequent life cycle tests of these concepts have been carried out that essentially have verified all of this earlier work. During the past decade, some of the mysteries involved in the active material of the nickel electrode have been resolved by careful research done at several laboratories. While attention has been paid to understanding and modeling abnormal nickel/hydrogen cell behaviors, not enough attention has been paid to the potassium ion content in these cells, and more recently, in batteries. Examining the potassium ion content of different portions of the cell or battery is a convenient way of following the conductivity, mass transport properties, and electrolyte volume in each of the cell or battery portions under consideration. Several of the consequences of solvent and solute changes within fuel cells have been well known for some time. However, only recently have these consequences been applied to nickel/hydrogen and nickel/cadmium cell designs. As a result of these studies, several unusual cell performance signatures can now be satisfactorily explained in terms of movement of the solvent and solute components in the electrolyte. This paper will review three general areas where the potassium ion content can impact the performance and life of nickel/hydrogen and nickel/cadmium cells. Sample calculations of the concentration or volume changes that can take place within operating cells will be presented. With the aid of an accurate model of an operating cell or battery, the impact of changes of potassium ion content within a potential cell design can be estimated. All three of these areas are directly related to the volume tolerance and pore size engineering aspects of the components used in the cell or battery design. The three areas follow. (i) The gamma phase uptake of potassium ion can result in a lowering of the electrolyte concentration. This leads to a higher electrolyte resistance as well as electrolyte diffusional limitations on the discharge rate. This phenomenon also impacts the response of the cell to a reconditioning cycle. (ii) The transport of water vapor from a warmer to a cooler portion of the cell or battery under the driving force of a vapor pressure gradient has already impacted cells when water vapor condenses on a colder cell wall. This paper will explore the convective and diffusive movement of gases saturated with water vapor from a warmer plate pack to a cooler one, both with and without liquid communication. (iii) The impact of low-level shunt currents in multicell configurations results in the net movement of potassium hydroxide from one part of the battery to another. This movement impacts the electrolyte volume/vapor pressure relationships within the cell or battery.

Techniques to improve the usability of nickel–hydrogen cells

The voltages at which the different charging and discharging peaks occur in a nickel electrode were investigated for a set of six representative electrodes. The impact of cycling temperature, electrolyte concentration, and cycling history on these different electrodes resulted in alterations in the usable capacities of the electrodes and/or the efficiency of the charging reaction. The cobalt level in the active material, the KOH concentration of the electrolyte, and the cycling temperature were all found to impact the position of the charging peaks of the beta and the gamma charging reactions. The position of the oxygen evolution characteristic was found to be mainly a function of the cycling temperature. The findings of these studies resulted in recommendations for selecting certain cell design factors and charging protocols that will lead to improved cycle lives and higher levels of usable cell capacities.

Understanding and managing capacity walkdown in nickel-hydrogen cells and batteries

The findings of a recent study that investigated the impact of cycling temperature, electrolyte concentration, cobalt content of the active material, and the cycling history of the electrode on the charging potential for the beta-phase and gamma-phase materials have been incorporated into earlier studies of the capacity walkdown phenomenon. Significant amounts of capacity walkdown were found to be associated with many of the life cycle testing programs carried out at the Navy facility in Crane, IN (USA). These findings are not only helpful for understanding the mechanisms associated with capacity walkdown, but will be useful in selecting cell design factors and cycling conditions that will allow the capacity loss to be held to an acceptable value.

Flooded Utilization and Electrochemical Voltage Spectroscopy Studies on Nickel Electrodes

A group of standard nickel electrodes were evaluated at different test temperatures and in different electrolyte concentrations. These production quality electrodes came from different backgrounds in terms of number of charge/discharge cycles, cycling temperature, electrolyte concentration, and cobalt level in the active material. The results of the matrix of tests using the flooded utilization (FU) technique demonstrated that capacity gains are available when cycling at lower temperatures and when higher concentrations of KOH are used as the electrolyte. Electrochemical voltage spectroscopy (EVS) scans were also taken for the complete matrix of tests. Since the cycling conditions used in the FU technique are much closer to those in actual cell cycling tests, they will be emphasized in this study in regard to the capacity trends. Comparative EVS scans were helpful in displaying the shifting potentials of the charging peaks of the active material relative to the oxygen evolution characteristics of these electrodes. The voltage span between the potential at which the active material is charged and the potential at which the co-evolution of oxygen becomes a significant parallel reaction determines the charging efficiency for the recharge step and is the root cause of the differences in useable electrode capacity.

A flooded—starved design for nickel—cadmium cells

Abstract It is suggested that some of the design technology used in alkaline fuel cells be applied to the design of nickel—cadmium aerospace cells. In particu